Furnaces are known which introduce heat for the purpose of re-flowing solder for the connection of components to circuit boards. The existing furnaces, however, suffer from a number of disadvantages, including high operation costs due to heat loss and the passage of ambient air into the open ends of the furnace's firing chamber. Also, the heating profile within the firing chamber is often inconsistent due to currents of heat escaping from the open ends of the firing chamber. Another disadvantage with the prior furnaces is that they cannot easily accommodate and transport different sized circuit boards having electronic components on both sides of the circuit boards.